Conventional boilers either fire tube type or water tube type are generally found with the following defects:
1. In order to sustain the high pressure of generated steam, all the construction parts should be designed with suitable thickness durable for such high steam pressure, which can not be reduced to be thinner so that the production cost of boiler will not be reduced. PA1 2. The heat conductivity of such thicker steel plates or tubes will be reduced so as to decrease the thermal efficiency of boiler. PA1 3. The outmost boiler shell should be insulated to prevent heat loss therefrom to increase insulation cost and maintenance problem for the insulated material as the boiler shell still carry an appreciated heat which should be well insulated. PA1 4. The boiler shell directly receives the high pressure of steam inside the boiler so that, once accidental explosion occurring, the outer broken scraps will directly and rapidly spread outwards to cause serious damage to the surroundings.
The present inventor has found these defects of conventional boilers and invented the present invention in view of the following specification and the accompanying drawings.